Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 14 2023 20:30:14 FOUS30 KWBC 142030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Fri Apr 14 2023 Day 1 Valid 16Z Fri Apr 14 2023 - 12Z Sat Apr 15 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA & THE MID-ATLANTIC/SOUTHERN APPALACHIANS... ....16Z Update... Very few changes needed for both ERO risk areas for this morning's update. ....South Florida... Typical afternoon sea breeze related convection is possible along the coast from Ft. Lauderdale to Miami today. Given the extreme sensitivity of this area to any additional rainfall, what little rain is possible today would still be enough to pose an isolated flash flooding risk. The Marginal Risk introduced overnight is unchanged for this midday update. ....The Carolinas to the southern Appalachians... Widely scattered convection is ongoing across central NC into southwest VA this morning. With additional daytime heating, expect MUCAPE values across the Marginal Risk area to approach 1,000 J/kg in the next few hours as the vertically stacked low over the Southeast slowly progresses northeast this afternoon. The low will cause temperatures to cool aloft, which will further destabilize the atmosphere as well as provide a forcing source for storms to develop along. Thus, expect scattered convection to develop over the Marginal Risk area over the next few hours, which while staying largely disorganized, has the potential to train since the low will continue moving very slowly. Local FFGs over central NC are a bit lower than the surrounding areas, with values to 1.5 inches in 3 hours. Given this afternoon=C2s storms have some potential to become strong, these values may be exceeded. Nominally higher chances for heavy rain are expected on the south and east facing slopes of the southern Appalachians as any convection tracks northwestward, but FFGs are also higher in these areas. Thus, the broad Marginal Risk is largely unchanged, other than including far western SC and far northeastern GA in the Marginal Risk area for this update. Wegman ....Previous Discussion... ....South Florida... Heavy rains yesterday caused some of the major roads in the Fort Lauderdale to Miami corridor to become flooded. Additional showers and thunderstorms are expected to fire up near the coast during the afternoon and evening hours, which will aggravate the already sensitive area. A Marginal Risk area was raised for the coastal/urban areas of southeast Florida. ....Mid-Atlantic and Central Appalachians... A deep mid/upper level low is expected to lift northward and weaken/open into Friday night. Moist easterly flow ahead of the advancing warm front will bring a surge of higher moisture through the region, characterized by PWs of up to 1.5 inches. With available instability (~1000 J/kg CAPE), locally intense rainfall will be possible across the Piedmont region. A secondary round of convection is possible during the afternoon/early evening associated with the main upper level shortwave moving overhead. The combination of steeper lapse rates and peak heating could bring another round of heavier showers and thunderstorms. Some of the hi-res guidance is depicting hourly rainfall of 1 to 1.5+ inches/hr across the Carolinas, as well as, eastern Kentucky, western Virginia and southern West Virginia. The 3-hr FFG are as low as 1.5 inches in the areas of higher elevation and around 2-2.5 inches elsewhere. The inherited Marginal Risk area was reshaped to trim out parts of South Carolina where the latest trends have lowered and then expanded east-west into western Kentucky, Tennessee and southern West Virginia where intense rainfall may quickly surpass local FFG and lead to isolated flooding concerns. Campbell Day 2 Valid 12Z Sat Apr 15 2023 - 12Z Sun Apr 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ....2030Z Update... ....Central Gulf Coast... In coordination with LIX/New Orleans, LA, MOB/Mobile, AL, and JAN/Jackson, MS forecast offices, a Slight Risk upgrade was introduced for this update. There has been a small, but nevertheless significant northward shift in the axis of heaviest rainfall expected on Saturday along the central Gulf Coast. As a cold front slowly moves east across the Plains, showers and thunderstorms are expected to break out along the coast, likely at least in part tied to the sea breeze, that will have over 2,000 J/kg of instability, and PWATs perhaps occasionally exceeding 1.75 inches, the storms will have plenty of energy to work with. Further, the flow ahead of the front will be rather weak, with storm motions around 10 kt. This will support training and back building thunderstorms, especially in the Slight Risk area where PWATs are greatest. There is still significant uncertainty as to where the heaviest rain will set up, and further if backbuilding convection will develop. Thus, at the moment confidence is low. However, much of the area picked up some rain yesterday which has saturated the soils, and there is enough potential for some of the backbuilding to impact urban areas such as New Orleans, Baton Rouge, and/or Mobile. ....Wisconsin and the U.P. of Michigan... The Marginal Risk area was expanded north to include all of WI and the western UP of MI with this update. Rapid snowmelt in northern WI and the U.P. of MI the past couple days has swollen local watersheds such that many of them are in minor to moderate flood stage. For much of the day Saturday into Sunday, light rain with embedded connective elements will track north across the area. The added rainfall will make exceeding the localized very low FFGs (in some areas as low as 1/4 inch in 6 hours) probable. MUCAPE values up to 1,000 J/kg are expected Saturday afternoon, which will support locally heavy rain. Thus, further upgrades are possible if expected rainfall amounts continue to increase ahead of the cold front. Wegman Day 3 Valid 12Z Sun Apr 16 2023 - 12Z Mon Apr 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF WISCONSIN AND THE UPPER PENINSULA OF MICHIGAN... ....Wisconsin and the U.P. of Michigan... Rainfall from Saturday will be ongoing into Sunday across the Marginal Risk area. As an area of low pressure develops along the front in response to a negatively tilting upper level trough ejecting out of the Plains and the influx of much colder air to its west, expect significantly more total precipitation on Sunday in this region as compared with Saturday. The complicating factor on Sunday will be how quickly the influx of cold air wrapping around the low will start changing precipitation over to snow and thus, rapidly ending the flash flooding threat. Western portions of the Marginal Risk area will see the rain change over earliest, however these areas are also starting off with the lowest FFGs, not even accounting for how much lower they will be after Saturday=C2s rain. Meanwhile, far eastern WI and the UP of MI may not see the changeover until after midnight Sunday night, which will give the rain more time to cause localized flash flooding before any changeover. The largest factor determining whether additional risk upgrades will be needed at this point is how the rivers and smaller creeks respond to the lesser amounts of rain on Saturday, with greater responses Saturday requiring more robust upgrades for Sunday. It will also be interesting to see how the CAMs resolve the rain on Sunday given the complete lack of instability in the Marginal Risk area by then. ....South Florida... A few of the longer range runs suggests a renewed round of afternoon convection may form near the Ft. Lauderdale area Sunday afternoon. While no Marginals were introduced for that possibility with this update, owing to the relatively few solutions suggesting convection will occur, that area will also need to be monitored for an upgrade depending on how much the area can recover and how much rain is expected by Sunday. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5Dm-jxAAXU633hzUjs7Ng55yCUoSKiEYFpeAdhJ6deXw= eU4IsxNOZujXPRdfuMwxpOLbAVrZvXzehWtZf4etIvbzn5k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Dm-jxAAXU633hzUjs7Ng55yCUoSKiEYFpeAdhJ6deXw= eU4IsxNOZujXPRdfuMwxpOLbAVrZvXzehWtZf4etJVQva5c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Dm-jxAAXU633hzUjs7Ng55yCUoSKiEYFpeAdhJ6deXw= eU4IsxNOZujXPRdfuMwxpOLbAVrZvXzehWtZf4et0YzmSZI$=20 $$ --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .